Thermische (E),(Z)-Isomerisierungen bei substituierten Propenylbenzolen

Abstract

The thermal isomerizations of (E)- and (Z)-3,5-dimethyl-2-(1'-propenyl)phenol ((E)- and (Z)-3),(E)- and (Z)-N-methyl-2-(1'-propenyl)anilin ((E)- and (Z)-4),(E)- and (Z)-3,5-dimethyl-2-(1'-propeny1)anilin ((E)- and (Z)-5), (E)- and (Z)-2-(1'- propeny1)mesitylene ((E)- and (Z)-6), (E)- and (Z)-2-(1'-butenyl)mesitylene ((E)- and (Z)-7), (E)-and (2)-2-(1'-propenyl)toluene ((E)- and (Z)-8), (E)- and (2)-4-(1'-propeny1)toluene ((E)- and (Z)-9) as well as of (E)- and (Z)-2-(2'-butenyl)mesitylene ((E)- and (Z)-10) in decane solution were studied (Scheme 2). Whereas the isomerization of the 2-propenylphenols (E)- and (Z)-3 occurs already between 130 and 150° (cf. Table 1), the isomerization of the 2-propenylanilins 4 and 5 takes place only at temperatures between 220 and 250° (cf. Tables 2 and 3). The activation values and the experiments using N-deuterated 4 (cf. Scheme 4) show that 2- propenylphenols and -anilins isomerize via sigmatropic [1,5]-hydrogen-shifts. For the isomerization of the methyl-substituted propenylbenzenes temperatures > 360° are required (cf. Tables 4 and 5). The activation values of the isomerization of (E)- and (Z)-6 and (E)- and (Z)-9 are in accord with those of other (E),(Z)-isomerizations which occur via vibrationally excited singlet biradicals (cf. Table 7). Nevertheless, thermal isomerization of 2'-d-(Z)-8 (cf. Scheme 6 )demonstrates that during the reaction deuterium is partially transfered into the ortho-methyl group, i.e. 1,5-hydrogen-shifts must have participated in isomerization of (E)- and (Z)-8 (cf. Scheme 8). Under the equilibrium conditions 2,4,6-trimethylindan (17) is formed slowly at 368° from (E)- and (Z)-6, very probably via a radical 1,4-hydrogen-shift (cf. Scheme 9). In a similar way 2-ethyl-4,6-dimethylindan (19; cf. Table 6) arises from (E)- and (Z)-7. Thermolysis of (E)- and (Z)-10 in decane solution at 367° results in almost no (E),(Z)-isomerization. At prolonged heating 19 and 2,5,7-trimethyl-1,2,3,4-tetrahydronaphthalene (20) are formed; these two products arise very likely from an intermolecular radical process (cf. Scheme 10).